Development of nuclear magnetic resonance (NMR) spectroscopy for biological diagnostics is well known in the prior art. It is understood that techniques for NMR spectroscopy rely upon identifying characteristic concentrations and distributions of protons in a test sample, which may be in vivo as well as in vitro, by subjecting the sample to pulses of electromagnetic energy while the same is positioned within a uniform magnetic field. A typical such pulse used to analyze protons is at 50 MHz for 10 microseconds, although frequencies and pulse widths will vary. Data characteristics of the proton population received while the sample is under the influence of the magnetic field yield valuable information about living systems without the use of invasive examination techniques and methods.
In one form, the device is portable and provided with means for receiving a portion of the body of a patient and exposing the portion of the body to a first fixed or biasing magnetic field and a second pulsed field generated by an energy source. Sensors are provided for sensing the rates of relaxation or energy release, commonly called the free induction decay (FID), so as to develop a spectrum. Analytical means are coupled to the sensors for receiving and analyzing the signals emitted, discriminating between various peaks, comparing the amplitude or height of various peaks, and normalizing the analysis by reference to a standard sample so as to obtain the concentration of constituents in the tested materials.
One of the principal components of the NMR instrument is the first fixed or biasing magnet for providing the first magnetic field. In portable devices, the biasing magnet is physically much smaller than the magnets used in the standard NMR machines. Another component is a coil for applying a second pulsed field to the test sample with an energy source and for sensing the energy released therefrom or the FID. A single coil or multiple coils can be used to apply the pulsed field and sense the released energy.
Useful application of NMR spectroscopy and imaging requires the apparatus to lock to the resonant frequency of the nucleus in order to obtain high resolution. Several methods have been developed over the years which include continuous wave (CW) and pulsed techniques. However, they all monitor the resonant frequency of the nuclei and adjust the magnetic field applied to the nuclei to maintain resonance. Since the nuclear resonant frequency is linearly related to the magnetic field, changes in the nuclear resonant frequency are equal to changes in the magnetic field. Historically, the operating frequency of the spectrometer is fixed by the use of a crystal oscillator. The resonant frequency of the nuclei is then compared to the fixed frequency of the oscillator to establish a difference frequency and the magnetic field is suitably altered according to the difference frequency to maintain the set difference. Such a device provides an explicit solution of the linear equation relating the magnetic field and the resonant frequency.
Several other related frequencies are generated in the transmitter to allow the receiver to track in frequency. Generally speaking, a dual conversion superheterodyne circuit is used in the prior art with a phase detector following the IF amplifier. Before the introduction of the phase detector at this point, an envelope detector such as a diode was used in this location. The output signal at this point is the FID signal. The phase detector merely serves as a second mixer and the receiver is thus classified as a dual conversion superheterodyne. The frequency of the FID is the difference between the exciter frequency and the nuclear resonant frequency. When the FID signal is zero, the circuit is tuned to resonance and the signal is synchronously detected to generate the amplitude that is equal to the exponential decay of the magnetic component of the nuclei. At this point, the amplitude of the generated signal provides little use as a lock. Additionally, any attempt to use it as a lock would be hampered by the DC drift and level shifts of direct coupled amplifiers. To produce a lock signal, an audio offset frequency is generated in the transmitter and a second phase detector is employed to generate an error sign`1 that is used to produce corresponding required changes in the magnetic field.
Although the prior art circuits are complicated, they have been successfully employed with electromagnets, superconducting magnets and perhaps some permanent magnets in the field of NMR spectroscopy. However, their application to ultrahigh energy product magnets is not feasible. The energy product of new rare earth magnetic materials such neodynium results in such large equivalent ampere-turns that it is difficult to augment the field with an additional electromagnetic field to substantially change the field. Additionally, the high energy product materials exhibit a large temperature coefficient which requires even greater correction. The problem is also aggravated in a highly homogeneous magnetic field design where multiple flux paths are utilized. Addition of ampere-turn windings to modulate the main magnetic field will alter the relationship to the side magnets and consequently change the gradients and the spectrometer resolution.
In order to overcome these disadvantages, the preferred embodiment of the present invention utilizes a frequency locking mechanism that depends upon a variable frequency. This is also an implicit solution to the linear equation relating field and frequency. Thus, the resonant frequency of the nuclei is compared to a variable frequency excitation that is adjusted to maintain a fixed offset frequency. One unique frequency value caused by this offset frequency is zero. The novel circuit utilizes a single conversion superheterodyne receiver, is much simpler than the dual conversion superheterodyne of the prior art and is available in integrated circuit form. It uses a phase detector after the IF amplifier in the receiver. The advantage of this system, besides the extensive use of integrated circuits and the reduction of components, is the resulting wider bandwidth of the loop filter and a corresponding faster response time. A voltage controlled oscillator tracks low frequency signals below the loop filter cut-off frequency and is unable to track frequencies above this value. As a result, during lock, the VCO assumes the spectral purity and phase noise characteristics of the nuclear reference signal around .+-.F.sub.n, the loop filter cut-off frequency, and thus regenerates the resonant frequency. This regenerated frequency can be translated by another phase locked loop to control a second spectrometer that will also track field changes of the same magnet. Since the FID of the second spectrometer is in the low audio range below F.sub.n as required by subsequent analog to digital converters, the increased loop bandwidth is beneficial.
Thus, it is an object of the present invention to provide an NMR circuit that uses a single conversion superheterodyne circuit.
It is also an object of the present invention to provide an NMR circuit that utilizes a frequency locking mechanism that depends upon a variable frequency.
It is still another object of the present invention to provide an NMR circuit in which the resonant frequency of the nuclei is compared to a variable frequency excitation that is adjusted to maintain a fixed offset frequency.
Still another object of the present invention is to provide an NMR circuit that can be used with ultrahigh energy product magnets such as neodynium.
Another object of the present invention is to provide an NMR circuit that reduces the number of required components and makes extensive use of integrated circuits.
It is also an object of the present invention to provide an NMR circuit that results in a wider bandwidth of the loop filter and has a corresponding faster response time than is available in the prior art.
It is another object of the present invention to provide a single conversion superheterodyne receiver generating a lock signal that is filtered in a feedback loop to obtain a signal that is sufficiently reduced in frequency so that it can vary the magnetic field to maintain lock.
It is yet another object of the present invention to provide a single conversion superheterodyne receiver generating a lock signal that is passed through a wide band filter in a feedback loop to obtain a frequency that is sufficiently high to reduce the phase noise of a voltage controlled oscillator and give greater accuracy in locking to the resonant frequency by causing the frequency of the voltage controlled oscillator to take on the high frequency characteristics of the nuclei signal frequency.